The Uber shader uses translucency to make skin look soft. That's countered by the "SSS" (which is a lie, it's actually volumetrics) consuming the light inside the skin layer. But Eevee can't do volumetrics so the skin has a translucency problem.
Yeah, I know. Missing features. So you fake it. It's basically Rule #1 of realtime graphics. If the computer/engine/language can't do it, find a cheap way to fake it.
For example, here's the result of a Difference operation between the second image and the third. If you multiply blend this image with the Eevee result, you'd get the Cycles result::
Looks like my gut was right about the AO pass, at least partially. Turns out you'd actually want to Add blend the Diffuse color (or maybe it's SSS color; it's a little hard to tell from here) to the AO before Multiplying it into the results. And then you'll still have these missing shadows around the eye sockets and jawline. Is that just the Diffuse lighting pass? Looking at the shadow under the chin, probably. I realize that you're not putting any effort into Eevee, but I'm surprised you turned off shadows entirely. Or maybe that's not the type of thing that's provided by Nvidia's standard shader language, and you'd have to actually put effort into implementing shadows yourself. Or at least combining your shader with an existing example shader that uses shadows. If that's the case, then fair enough. You did your best.
Here's something that you personally might find a little more useful. It's the result of a difference operation between the first image and the second:
I'm not sure what this actually is, but to me it sort of looks like if you took the bump map, did a High Pass on it, and added the diffuse map to it, you'd get something not entirely unlike this. Oh, and I noticed you need to Overlay at 50% or so, not Multiply. Looks like different missing data needs to be blended differently, which makes sense. No, wait. I just realized that you're not using the shape morphs in Blender. Everything that looks like High Pass could be a side-effect of the morph changing shape, thereby nudging the skin details until they're out of sync with each other.
I notice the morph doesn't affect the ear, though...
Result * ( ( ( Bump * (1 - Layer Weight ) ) + Diffuse Color) * 0.5) * 1.5. Final answer.
And, of course, applying both those effects at once could turn the Eevee result into a rough approximation of the Iray result.
(In theory, anyway. In practice, this is only one point of data from a very specific test-case. Lessons gleaned from this exercise might not apply in the general case, etc. Which means, at best, if you blindly messed with the shader based on these comparisons, you'd end up with some sort of frankenstien shader that only works on skin, or even a very specific range of skin tones, and you might need to massage the input bitmaps so they more closely resemble whatever conventions the developers of Sophie's original Skin were using when they designed their skin maps. Maybe replace some of those floats with variables so the artist can tweak it with sliders at the Material level. But, that's realtime graphics development for you. Gotta fake it when you literally can't make it.)
The broader point I'm trying to make is, when you hit the end of the road chasing realism, you can often push it further by embracing hyperrealism-- the pursuit of what it
looks like, not what it
is.
(That's kind of what the Iray Ubershader is, in the first place, when you think about it. A hackey cheat for approximating reality. If it were somehow
literally "photo-real lighting," using it would begin and end with images scanned-in from real people and objects. There would be no need for all the colors and sliders and extra secular models to choose from. Instead, they wisely chose to give the artists wiggle room so they could fake various effects for which no map exists, or modulate existing maps for new use-cases.)
